Boron nitride (hereinafter referred to as BN) is one of rare ceramics having excellent characteristics, such as electric insulation, thermal conductivity, resistance to corrosion, resistance to thermal shock and lubricating property, and adapted to be easily machined. For this reason, it is widely used as a material for various containers for melting metals therein, electrically insulating materials and heat conductor materials used at high temperature, for which the aforementioned properties are required.
Since BN is hardly sintered, sintered bodies of BN are generally produced by the hot press method (sintering under pressure). In practice, a pressure more than 100 kg/cm.sup.2 must be applied at a temperature of from 1500 to 2300.degree. C., which necessitates the use of graphite dies to thus make it impossible to form a large molded body. There is also a problem that the efficiency of mass production of articles of complicated shape is lowered.
Even in press sintering methods, such as in the hot press method, it is a common practice to add a sintering aid, such as boron oxides or borates of alkaline earth metals. However, because such a sintering aid remains in the hot press sintered body, it causes to deteriorate characteristics inherent to BN such as lubricating property, electrically insulating property, thermal conductivity and resistance to thermal shock at elevated temperatures, and thus the sintering aid is deemed a harmful component contained in the final sintered body. Moreover, since the BN powder has hexagonal crystallite lamellar structures which tend to be easily oriented at the molding step, the hot press sintered body thereof has anisotropic properties and the use thereof is limited sometimes. Although it has been tried, for example, to effect rubber press molding at a pre-molding step and then the pre-molded mass is put into a graphite mold and sintered under the condition of limiting free expansion (Japanese Laid-Open Patent publication No. 132563/1986) in order to produce an isotropic sintered body, such a process is low in production efficiency in addition to the aforementioned problem caused by the sintering aid.
Although various pressureless sintering processes have been tried to solve these problems, a sintered body of high purity and high strength exhibiting the properties of BN a sufficient extent has not yet been produced. For example, Japanese Patent Publication No. 12547/1963 discloses a process wherein boric anhydride is added to BN and then pressurelessly sintered. However, the product produced thereby has little practical utility since the content of BN is as high as 95 to 99 wt. % but the strength thereof is as low as 5 kg/cm.sup.2. On the other hand, Japanese Patent publication No. 38047/1972 describes a process in which SiO.sub.2 and B.sub.2 O.sub.3 are added to BN. However, although the product produced thereby has high strength of from 350 to 700 kg/cm.sup.2, the content of BN is as low as in the order of 30 to 70 wt. %, leading to the failure of satisfactory exhibition of properties of BN, and particularly the product is not suited for an insulating material or a crucible for melting metals which must have an electrically insulating property, thermal conductivity, resistance to corrosion and resistance to thermal shock.
As the electric equipments are progressively smaller and highly integrated, problems arise in conduction and radiation of heat generated from the circuit elements and there is a demand for an electrically insulated heat sink (hereinafter referred simply to as "heat sink") having sufficient insulating property, high thermal conductivity and high mechanical strength.
Inexpensive alumina has conventionally been used for the heat sink. However, since thermal conductivity of alumina is not sufficiently high, BN has been adopted in lieu of alumina in recent years to cope with the radiation of heat from a high power electronic equipment. However, since BN is sintered mainly through the hot press method, the sintered ingot must be post-machined to have a desired shape which requires labor and poses a problem that the insulation resistance and thermal conductivity are lowered due to the addition of a sintering aid. It is also another big problem that the insulating resistance is lowered by absorption of moisure by the sintering aid in the sintered body. For example, a method of coating a resin on a sintered body is disclosed (Japanese Laid-Open Patent Publication No. 116181/1984) in order to improve the anti-hygroscopic property of the sintered body, but there is a problem that a layer of low thermal conductivity is formed on the surface to lower the thermal conductivity although the anti-hygroscopic property is improved.
For these reasons, there is a demand for the provision of a pressureless sintered body of BN having high purity and high strength and a process for producing the same which is excellent in characteristics inherent to BN such as electrically insulating property, thermal conductivity, resistance to corrosion, resistance to thermal shock and lubricating property without adding any sintering aid.
The object of this invention is to improve the strength and other properties of a pressureless sintered body of BN and to provide a presssureless sintered body of BN having high purity and high strength and having satisfactory thermal conductivity and electrically insulating property, which have not been obtainable by the conventional technology.